Tungsten oxide processing

ABSTRACT

Methods of selectively etching tungsten oxide relative to tungsten, silicon oxide, silicon nitride and/or titanium nitride are described. The methods include a remote plasma etch using plasma effluents formed from a fluorine-containing precursor in combination with ammonia (NH 3 ). Plasma effluents from the remote plasma are flowed into a substrate processing region where the plasma effluents react with the tungsten oxide. The plasmas effluents react with exposed surfaces and selectively remove tungsten oxide while very slowly removing other exposed materials. Increasing a flow of ammonia during the process removes a typical skin of tungsten oxide having higher oxidation coordination number first and then selectively etching lower oxidation tungsten oxide. In some embodiments, the tungsten oxide etch selectivity results partly from the presence of an ion suppression element positioned between the remote plasma and the substrate processing region.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Prov. Pat. App. No. 61/896,741 filed Oct. 29, 2013, and titled “TUNGSTEN OXIDE PROCESSING” by Liu et al., which is hereby incorporated herein in its entirety by reference for all purposes.

FIELD

Embodiments of the invention relate to cleaning and selectively etching tungsten oxide.

BACKGROUND

Integrated circuits are made possible by processes which produce intricately patterned material layers on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed material. Chemical etching is used for a variety of purposes including transferring a pattern in photoresist into underlying layers, thinning layers or thinning lateral dimensions of features already present on the surface. Often it is desirable to have an etch process which etches one material faster than another helping e.g. a pattern transfer process proceed. Such an etch process is said to be selective of the first material. As a result of the diversity of materials, circuits and processes, etch processes have been developed that selectively remove one or more of a broad range of materials.

Dry etch processes are often desirable for selectively removing material from semiconductor substrates. The desirability stems from the ability to gently remove material from miniature structures with minimal physical disturbance. Dry etch processes also allow the etch rate to be abruptly stopped by removing the gas phase reagents. Some dry-etch processes involve the exposure of a substrate to remote plasma by-products formed from one or more precursors. For example, remote plasma excitation of ammonia and nitrogen trifluoride enables silicon oxide to be selectively removed from a patterned substrate when the plasma effluents are flowed into the substrate processing region. Remote plasma etch processes have recently been developed to selectively remove a variety of dielectrics relative to one another. However, fewer dry-etch processes have been developed to selectively remove metal oxides.

Methods are needed to selectively etch metal oxides using dry etch processes.

SUMMARY

Methods of selectively etching tungsten oxide relative to tungsten, silicon oxide, silicon nitride and/or titanium nitride are described. The methods include a remote plasma etch using plasma effluents formed from a fluorine-containing precursor in combination with ammonia (NH₃). Plasma effluents from the remote plasma are flowed into a substrate processing region where the plasma effluents react with the tungsten oxide. The plasmas effluents react with exposed surfaces and selectively remove tungsten oxide while very slowly removing other exposed materials. Increasing a flow of ammonia during the process removes a typical skin of tungsten oxide having higher oxidation coordination number first and then selectively etching lower oxidation tungsten oxide. In some embodiments, the tungsten oxide etch selectivity results partly from the presence of an ion suppression element positioned between the remote plasma and the substrate processing region.

Embodiments include methods of etching tungsten oxide. The methods include flowing a fluorine-containing precursor and ammonia (NH₃) into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead while forming a remote plasma in the remote plasma region to produce plasma effluents. The methods further include etching the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead.

Embodiments include methods of etching tungsten oxide. The methods include flowing a fluorine-containing precursor and ammonia (NH₃) into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead. The fluorine-containing precursor and the ammonia are flowed at a first flow rate ratio (ammonia:fluorine-containing precursor). The methods further include forming a remote plasma in the remote plasma region to produce plasma effluents from the fluorine-containing precursor and the ammonia (NH₃). The methods further include etching a first portion of the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead. The methods further include increasing flow rate ratio (ammonia:fluorine-containing precursor) to a second flow rate ratio which is greater than the first flow rate ratio. The methods further include etching a second portion of the tungsten oxide from the substrate by continuing to flow plasma effluents into the substrate processing region.

Embodiments include methods of etching tungsten oxide. The methods further include flowing nitrogen trifluoride (NF₃) at a nitrogen trifluoride flow rate and ammonia (NH₃) at a first ammonia flow rate into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead. The methods further include forming a remote plasma in the remote plasma region to produce plasma effluents from the nitrogen trifluoride and the ammonia. The methods further include etching a first portion of the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead. The methods further include increasing the flow rate of the ammonia from the first ammonia flow rate to a second ammonia flow rate. The methods further include etching a second portion of the tungsten oxide from the substrate by continuing to flow plasma effluents into the substrate processing region.

Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed embodiments. The features and advantages of the disclosed embodiments may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification.

DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the disclosed embodiments may be realized by reference to the remaining portions of the specification and the drawings.

FIG. 1 is a flow chart of a tungsten oxide selective etch process according to embodiments.

FIG. 2 is a flow chart of a tungsten oxide selective etch process according to embodiments.

FIG. 3A shows a substrate processing chamber according to embodiments.

FIG. 3B shows a showerhead of a substrate processing chamber according to embodiments.

FIG. 4 shows a substrate processing system according to embodiments.

In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

DETAILED DESCRIPTION

Methods of selectively etching tungsten oxide relative to tungsten, silicon oxide, silicon nitride and/or titanium nitride are described. The methods include a remote plasma etch using plasma effluents formed from a fluorine-containing precursor in combination with ammonia (NH₃). Plasma effluents from the remote plasma are flowed into a substrate processing region where the plasma effluents react with the tungsten oxide. The plasmas effluents react with exposed surfaces and selectively remove tungsten oxide while very slowly removing other exposed materials. Increasing a flow of ammonia during the process removes a typical skin of tungsten oxide having higher oxidation coordination number first and then selectively etching lower oxidation tungsten oxide. In some embodiments, the tungsten oxide etch selectivity results partly from the presence of an ion suppression element positioned between the remote plasma and the substrate processing region.

In order to better understand and appreciate the invention, reference is now made to FIG. 1 which is a flow chart of a tungsten oxide selective etch process 100 according to embodiments. The tungsten oxide may have a variety of stoichiometries which may be determined by the method of forming the tungsten oxide. Prior to the first operation, tungsten oxide is formed on a substrate. The tungsten oxide may be in the form of a blanket layer on the substrate or it may reside in discrete portions of a patterned substrate surface. In either case, the tungsten oxide forms exposed surfaces of the surface of the substrate. The substrate is then delivered into a processing region (operation 110). In another embodiment, the tungsten oxide may be formed after delivering the substrate to the processing region, for example, by treating exposed portions of tungsten to a reactive oxygen source.

A flow of nitrogen trifluoride is introduced into a plasma region separate from the processing region (operation 120). Other sources of fluorine may be used to augment or replace the nitrogen trifluoride. In general, a fluorine-containing precursor may be flowed into the plasma region and the fluorine-containing precursor comprises at least one precursor selected from the group consisting of atomic fluorine, diatomic fluorine, bromine trifluoride, chlorine trifluoride, nitrogen trifluoride, hydrogen fluoride, sulfur hexafluoride and xenon difluoride. Ammonia (NH₃) is also flowed into the plasma region (operation 125) where it is simultaneously excited in a plasma along with the nitrogen trifluoride. The flow rate of the ammonia may be less than the flow rate of the nitrogen trifluoride (e.g. between 75% and 98% of the nitrogen trifluoride flow rate) in order to optimize the removal rate of tungsten oxide. Specific value ranges for the flow rates will be discussed shortly.

The separate plasma region may be referred to as a remote plasma region for all etch processes described herein and may be within a distinct module from the processing chamber or a compartment within the processing chamber. The separate plasma region may is fluidly coupled to the substrate processing region by through-holes in a showerhead disposed between the two regions. The hardware just described may also be used in all processes discussed herein. The remote plasma region may be a capacitively-coupled plasma region, in embodiments, and may be disposed remote from the substrate processing region of the processing chamber. For example, the capacitively-coupled plasma region (and the remote plasma region in general) may be separated from the substrate processing region by the showerhead.

The plasma effluents formed in the remote plasma region are then flowed into the substrate processing region (operation 130). Tungsten oxide on the substrate is selectively etched (operation 135) such that tungsten oxide may be removed more rapidly than a variety of other materials. The selective etch in all examples disclosed herein may etch tungsten oxide significantly faster than one or more of tungsten, silicon oxide, silicon nitride, and titanium nitride in embodiments. Such a process may have broad-based utility, but may find clear utility in removing a layer of tungsten oxidation which forms on a layer of tungsten when ashing a patterned tungsten surface to clean off photoresist residue. Etch process 100 may involve maintenance of an atomic flow ratio of fluorine (F) to hydrogen (H) in order achieve high etch selectivity of tungsten oxide. The copresence of fluorine and hydrogen is thought to enable two reactions to proceed: (1) one which consumes ammonia (NH₃) to convert the tungsten oxide to tungsten by shedding moisture through the exhaust system and (2) another reaction which consumes fluorine to remove tungsten in the form of exhausted tungsten hexafluoride (WF₆). Despite the hypothetical second reaction, tungsten oxide selective etch process 100 is highly selective of tungsten oxide over tungsten as long as the ammonia to nitrogen-trifluoride flow rate ratio is maintained as outlined herein. The reactive chemical species and any process effluents are removed from the substrate processing region and then the substrate is removed from the processing region (operation 145).

The substrate may be a patterned substrate which further includes one or more exposed regions of tungsten, silicon oxide or titanium nitride. The etch selectivity (tungsten oxide:tungsten) of the process of FIG. 1, as well as the dry etch process to be discussed with reference to FIG. 2, may be greater than or about 10:1, greater than or about 20:1, greater than or about 50:1, or greater than or about 100:1 according to embodiments. Tungsten oxide selective etch process 100 and the other tungsten oxide selective etch processes discussed herein, display etch selectivities of tungsten oxide relative to a variety of other materials. The etch selectivity of tungsten oxide relative to silicon oxide may be greater than or about 15:1, greater than or about 40:1, greater than or about 75:1 or greater than or about 150:1 according to embodiments. The etch selectivity of tungsten oxide relative to silicon nitride may be greater than or about 20:1, greater than or about 50:1, greater than or about 100:1 or greater than or about 200:1 in embodiments. The etch selectivity of tungsten oxide relative to titanium nitride may be greater than or about 20:1, greater than or about 50:1, greater than or about 100:1 or greater than or about 200:1 according to embodiments.

In each remote plasma described herein, the flows of the precursors into the remote plasma region may further include one or more relatively inert gases such as He, N₂, Ar. The inert gas can be used to improve plasma stability, ease plasma initiation, and improve process uniformity. Argon is helpful, as an additive, to promote the formation of a stable plasma. Process uniformity is generally increased when helium is included. These additives are present in embodiments throughout this specification. Flow rates and ratios of the different gases may be used to control etch rates and etch selectivity.

Using ammonia (NH₃) instead of hydrogen (H₂) to supply hydrogen to the remote plasma region results in a much broader process window for tungsten oxide selective etch process 100. The flow rate of the ammonia as a percentage of the flow rate of the nitrogen trifluoride may be between 75% and 100%, between 80% and 99% or preferably between 83% and 97% according to embodiments. Operating tungsten oxide selective etch processes above these ranges results in a low tungsten oxide etch rate, whereas operating the etch processes described herein below these ranges sacrifices etch selectivity compared to tungsten (i.e. the tungsten etch rate rises quickly). The preferred range for flow rate ratios spans fourteen percentage points (83%-97%), which is about a three times larger process window than when hydrogen (H₂) was used in place of the ammonia.

In embodiments, the fluorine-containing gas (e.g. NF₃) is supplied at a flow rate of between about 25 sccm (standard cubic centimeters per minute) and 400 sccm, NH₃ at a flow rate of between about 20 sccm and 350 sccm, He at a flow rate of between about 0 slm (standard liters per minute) and 3 slm, and Ar at a flow rate of between about 0 slm and 3 slm. Generally speaking, the fluorine-containing precursor may be supplied at a flow rate between about 25 sccm and about 400 sccm, between about 50 sccm and about 300 sccm, between about 75 sccm and about 200 sccm or between about 125 sccm and about 175 sccm according to embodiments. Ammonia (NH₃) may be supplied at a flow rate between about 20 sccm and about 350 sccm, between about 30 sccm and about 300 sccm, between about 50 sccm and about 200 sccm or between about 125 sccm and about 145 sccm in embodiments.

Tungsten oxide selective etch process 100 (and the other etch processes described herein) includes applying energy to the fluorine-containing precursor and the ammonia while they flow through the remote plasma region to generate the plasma effluents. The plasma may be generated using known techniques (e.g., radio frequency excitations, capacitively-coupled power, and inductively-coupled power). In an embodiment, the energy is applied using a capacitively-coupled plasma unit. The remote plasma source power may be between about 10 watts and about 2500 watts, between about 30 watts and about 2000 watts, between about 100 watts and about 1500 watts, or between about 300 watts and about 1000 watts according to embodiments. In a preferred embodiment, the remote plasma source power may be between 400 watts and 800 watts, which represents a process window of 50%. Using hydrogen (H₂) in place of ammonia resulted in a process window of between 30 watts and 50 watts for a process window of about 40%. In absolute terms, the process window using ammonia is 400 watts wide as opposed to 20 watts, which represents a desirable increase in process flexibility.

The pressure in the substrate processing region is about the same as the pressure in the substrate processing region, according to embodiments, in all tungsten oxide selective etch processes described herein. The pressure in the remote plasma region and also in the substrate processing region is between about 0.1 Torr and about 50 Torr, between about 0.5 Torr and about 20 Torr or preferably between about 1 Torr and about 10 Torr in embodiments. Pressures closer to the higher end of the preferred embodiment (e.g. between 5 Torr and 10 Ton) results in higher selectivities compared to the lower end of the preferred embodiment.

The temperature of the substrate during etch processes described herein may be between about 30° C. and about 300° C. in general. In embodiments, the temperature of the substrate during the etches described in this section may be greater than or about 40° C., greater than or about 60° C., greater than or about 80° C. or greater than or about 100° C. The substrate temperatures may be less than or about 250° C., less than or about 200° C., less than or about 180° C., and may be between about 100° C. and about 160° C. in embodiments.

In order to further appreciate the invention, reference is now made to FIG. 2 which is a flow chart of a tungsten oxide selective etch process 200 according to embodiments. The various traits and process parameters discussed with reference to FIG. 1 will not be repeated here except when they deviate from those traits and process parameters. Prior to the first operation, tungsten oxide is formed on a substrate. The substrate is then delivered into a processing region (operation 210). The tungsten oxide may alternatively be formed after delivering the substrate to the processing region. In either case, the tungsten oxide may be formed by treating exposed portions of tungsten to a reactive oxygen source.

Flows of nitrogen trifluoride and ammonia (NH₃) are introduced into the remote plasma region (operation 220). Again, other sources of fluorine may be used to augment or replace the nitrogen trifluoride. In general, the fluorine-containing precursor may comprises at least one precursor selected from the group consisting of atomic fluorine, diatomic fluorine, bromine trifluoride, chlorine trifluoride, nitrogen trifluoride, hydrogen fluoride, sulfur hexafluoride and xenon difluoride. The flow rate of the ammonia may be less than the flow rate of the nitrogen trifluoride (e.g. between 75% and 92% of the nitrogen trifluoride flow rate) in order to optimize the removal rate of a thin layer of tungsten oxide having a higher oxygen proportion than underlying “bulk” tungsten oxide. The thin tungsten oxide layer has a W:O atomic ratio of roughly 1:3 which benefits from a slightly lower ammonia:NF₃ ratio. For comparison, the bulk tungsten oxide ratio has a W:O atomic ratio of roughly 1:2.

The plasma effluents formed in the remote plasma region are then flowed into the substrate processing region (operation 225). Tungsten oxide on the substrate is selectively etched (operation 230) such that thin tungsten oxide (e.g. WO₃) may be removed more rapidly than a variety of other materials. This process may be referred to as tungsten oxide “break-through” herein. The selective etch of the WO₃ layer (as well as the bulk WO₂ layer etched later in etch process 200) may etch tungsten oxide significantly faster than one or more of tungsten, silicon oxide, silicon nitride, and titanium nitride according to embodiments. Exemplary selectivity ranges were given previously in the first example.

The flow rate of the ammonia is increased in operation 233. The increased flow rate of ammonia while the nitrogen trifluoride flow rate is held roughly constant tailors the process for the removal of bulk tungsten oxide (e.g. WO₂). The increase in flow rate of the ammonia maintains the selectivities, described earlier, of the tungsten oxide relative to the other materials which may be present on the patterned substrate. In general, the flow rate ratio of the ammonia to the nitrogen trifluoride is increased. For example, the flow rate of the nitrogen trifluoride may be reduced to achieve the same beneficial transition from etching the thin outer tungsten oxide layer (e.g. WO₃) to etching the bulk tungsten oxide layer (e.g. WO₂). For example, the flow rate of the ammonia may be between about 83% and 90% of the nitrogen trifluoride during removal of the WO₃ layer. In contrast, the flow rate of the ammonia may be between about 90% and 97% of the nitrogen trifluoride during removal of the bulk WO₂ layer. The flow rate ratio (NH₃:NF₃) may be increased by at least 1%, 2%, 3% or preferably 5% from operation 220 to after operation 233. Increasing the flow rate ratio may occur essentially in a single discrete step according to embodiments. The bulk tungsten oxide is etched in operation 235.

Following removal of tungsten oxide, the reactive chemical species and any process effluents are removed from the substrate processing region and then the substrate is removed from the processing region (operation 245). The etch selectivity ratios for the process just discussed with reference to FIG. 2 contain the same embodiments recited above with reference to FIG. 1.

In embodiments, the fluorine-containing gas (e.g. NF₃) is supplied at a flow rate of between about 25 sccm (standard cubic centimeters per minute) and 400 sccm, NH₃ at a flow rate of between about 20 sccm and 350 sccm, He at a flow rate of between about 0 slm (standard liters per minute) and 3 slm, and Ar at a flow rate of between about 0 slm and 3 slm. Generally speaking, the fluorine-containing precursor may be supplied at a flow rate between about 25 sccm and about 400 sccm, between about 50 sccm and about 300 sccm, between about 75 sccm and about 200 sccm or preferably between about 125 sccm and about 175 sccm according to embodiments. During operation 220, ammonia (NH₃) may be supplied at a flow rate between about 20 sccm and about 300 sccm, between about 30 sccm and about 250 sccm, between about 50 sccm and about 175 sccm or preferably between about 125 sccm and about 135 sccm in embodiments. Following operation 233, ammonia (NH₃) may be supplied at a flow rate between about 30 sccm and about 350 sccm, between about 50 sccm and about 300 sccm, between about 75 sccm and about 200 sccm or preferably between about 135 sccm and about 145 sccm in embodiments.

As indicated previously, the remote plasma source power may be applied capacitively and may be between about 10 watts and about 2500 watts, between about 30 watts and about 2000 watts, between about 100 watts and about 1500 watts, between about 300 watts and about 1000 watts or preferably between about 400 watts and 800 watts. The pressure in the substrate processing region and/or the remote plasma region may be between about 0.1 Torr and about 50 Torr, between about 0.5 Torr and about 20 Torr or preferably between about 1 Torr and about 10 Torr in embodiments. The pressure may be between about 5 Torr and about 10 Torr in a preferred embodiment. The temperature of the substrate may be between about 30° C. and about 300° C., between about 40° C. and about 250° C., between about 60° C. and about 180° C., between about 80° C. and about 150° C. or preferably between about 100° C. and about 160° C. in embodiments.

In embodiments, an ion suppressor as described in the exemplary equipment section may be used to provide radical and/or neutral species for selectively etching substrates. The ion suppressor may also be referred to as an ion suppression element. In embodiments, for example, the ion suppressor is used to filter fluorine and hydrogen containing plasma effluents to selectively etch tungsten oxide. The ion suppressor may be included in each exemplary process described herein. Using the plasma effluents, an etch rate selectivity of tungsten oxide to a wide variety of materials may be achieved.

The ion suppressor may be used to provide a reactive gas having a higher concentration of radicals than ions. The ion suppressor functions to dramatically reduce or substantially eliminate ionically charged species traveling from the plasma generation region to the substrate. The electron temperature may be measured using a Langmuir probe in the substrate processing region during excitation of a plasma in the remote plasma region on the other side of the ion suppressor. In embodiments, the electron temperature may be less than 0.5 eV, less than 0.45 eV, less than 0.4 eV, or preferably less than 0.35 eV. These extremely low values for the electron temperature are enabled by the presence of the showerhead and/or the ion suppressor positioned between the substrate processing region and the remote plasma region. Uncharged neutral and radical species may pass through the openings in the ion suppressor to react at the substrate. Because most of the charged particles of a plasma are filtered or removed by the ion suppressor, the substrate is not necessarily biased during the etch process. Such a process using radicals and other neutral species can reduce plasma damage compared to conventional plasma etch processes that include sputtering and bombardment. The ion suppressor helps control the concentration of ionic species in the reaction region at a level that assists the process. Embodiments of the present invention are also advantageous over conventional wet etch processes where surface tension of liquids can cause bending and peeling of small features.

Generally speaking, the processes described herein may be used to etch films which contain tungsten and oxygen (and not just any specific example of stoichiometric tungsten oxide). The remote plasma etch processes may remove tungsten oxide which includes an atomic concentration of about 20% or more tungsten and about 60% or more oxygen according to embodiments. The tungsten oxide may consist only of tungsten and oxygen, allowing for small dopant concentrations and other undesirable or desirable minority additives. Tungsten oxide may have roughly one of the following atomic ratios (W:0) in embodiments: 2:3, 1:2, 1:3, 1:4 or 2:5. The tungsten oxide may contain more than 25% or 30% tungsten and may contain more than 65%, 70% or 75% oxygen in embodiments. Upper limits on atomic concentration may be combined with lower limits according to embodiments.

Additional process parameters are disclosed in the course of describing an exemplary processing chamber and system.

Exemplary Processing System

Processing chambers that may implement embodiments of the present invention may be included within processing platforms such as the CENTURA® and PRODUCER® systems, available from Applied Materials, Inc. of Santa Clara, Calif.

FIG. 3A is a substrate processing chamber 1001 according to embodiments. A remote plasma system 1010 may process a fluorine-containing precursor and/or a hydrogen-containing precursor which then travels through a gas inlet assembly 1011. Two distinct gas supply channels are visible within the gas inlet assembly 1011. A first channel 1012 carries a gas that passes through the remote plasma system 1010 (RPS), while a second channel 1013 bypasses the remote plasma system 1010. Either channel may be used for the fluorine-containing precursor, in embodiments. On the other hand, the first channel 1012 may be used for the process gas and the second channel 1013 may be used for a treatment gas. The lid (or conductive top portion) 1021 and a perforated partition 1053 are shown with an insulating ring 1024 in between, which allows an AC potential to be applied to the lid 1021 relative to perforated partition 1053. The AC potential strikes a plasma in chamber plasma region 1020. The process gas may travel through first channel 1012 into chamber plasma region 1020 and may be excited by a plasma in chamber plasma region 1020 alone or in combination with remote plasma system 1010. If the process gas (the fluorine-containing precursor) flows through second channel 1013, then only the chamber plasma region 1020 is used for excitation. The combination of chamber plasma region 1020 and/or remote plasma system 1010 may be referred to as a remote plasma system herein. The perforated partition (also referred to as a showerhead) 1053 separates chamber plasma region 1020 from a substrate processing region 1070 beneath showerhead 1053. Showerhead 1053 allows a plasma present in chamber plasma region 1020 to avoid directly exciting gases in substrate processing region 1070, while still allowing excited species to travel from chamber plasma region 1020 into substrate processing region 1070.

Showerhead 1053 is positioned between chamber plasma region 1020 and substrate processing region 1070 and allows plasma effluents (excited derivatives of precursors or other gases) created within remote plasma system 1010 and/or chamber plasma region 1020 to pass through a plurality of through-holes 1056 that traverse the thickness of the plate. The showerhead 1053 also has one or more hollow volumes 1051 which can be filled with a precursor in the form of a vapor or gas (such as a fluorine-containing precursor) and pass through small holes 1055 into substrate processing region 1070 but not directly into chamber plasma region 1020. Showerhead 1053 is thicker than the length of the smallest diameter 1050 of the through-holes 1056 in this embodiment. The length 1026 of the smallest diameter 1050 of the through-holes may be restricted by forming larger diameter portions of through-holes 1056 part way through the showerhead 1053 to maintain a significant concentration of excited species penetrating from chamber plasma region 1020 to substrate processing region 1070. The length of the smallest diameter 1050 of the through-holes 1056 may be the same order of magnitude as the smallest diameter of the through-holes 1056 or less in embodiments.

Showerhead 1053 may be configured to serve the purpose of an ion suppressor as shown in FIG. 3A. Alternatively, a separate processing chamber element may be included (not shown) which suppresses the ion concentration traveling into substrate processing region 1070. Lid 1021 and showerhead 1053 may function as a first electrode and second electrode, respectively, so that lid 1021 and showerhead 1053 may receive different electric voltages. In these configurations, electrical power (e.g., RF power) may be applied to lid 1021, showerhead 1053, or both. For example, electrical power may be applied to lid 1021 while showerhead 1053 (serving as ion suppressor) is grounded. The substrate processing system may include a RF generator that provides electrical power to the lid and/or showerhead 1053. The voltage applied to lid 1021 may facilitate a uniform distribution of plasma (i.e., reduce localized plasma) within chamber plasma region 1020. To enable the formation of a plasma in chamber plasma region 1020, insulating ring 1024 may electrically insulate lid 1021 from showerhead 1053. Insulating ring 1024 may be made from a ceramic and may have a high breakdown voltage to avoid sparking. Portions of substrate processing chamber 1001 near the capacitively-coupled plasma components just described may further include a cooling unit (not shown) that includes one or more cooling fluid channels to cool surfaces exposed to the plasma with a circulating coolant (e.g., water).

In the embodiment shown, showerhead 1053 may distribute (via through-holes 1056) process gases which contain fluorine, hydrogen and/or plasma effluents of such process gases upon excitation by a plasma in chamber plasma region 1020. In embodiments, the process gas introduced into the remote plasma system 1010 and/or chamber plasma region 1020 may contain fluorine (e.g. F₂, NF₃ or XeF₂). The process gas may also include a carrier gas such as helium, argon, nitrogen (N₂), etc. Plasma effluents may include ionized or neutral derivatives of the process gas and may also be referred to herein as radical-fluorine and/or radical-hydrogen referring to the atomic constituent of the process gas introduced.

Through-holes 1056 are configured to suppress the migration of ionically-charged species out of the chamber plasma region 1020 while allowing uncharged neutral or radical species to pass through showerhead 1053 into substrate processing region 1070. These uncharged species may include highly reactive species that are transported with less-reactive carrier gas by through-holes 1056. As noted above, the migration of ionic species by through-holes 1056 may be reduced, and in some instances completely suppressed. Controlling the amount of ionic species passing through showerhead 1053 provides increased control over the gas mixture brought into contact with the underlying wafer substrate, which in turn increases control of the deposition and/or etch characteristics of the gas mixture. For example, adjustments in the ion concentration of the gas mixture can alter the etch selectivity (e.g., the tungsten oxide:tungsten etch ratio).

In embodiments, the number of through-holes 1056 may be between about 60 and about 2000. Through-holes 1056 may have a variety of shapes but are most easily made round. The smallest diameter 1050 of through-holes 1056 may be between about 0.5 mm and about 20 mm or between about 1 mm and about 6 mm in embodiments. There is also latitude in choosing the cross-sectional shape of through-holes, which may be made conical, cylindrical or combinations of the two shapes. The number of small holes 1055 used to introduce unexcited precursors into substrate processing region 1070 may be between about 100 and about 5000 or between about 500 and about 2000 in different embodiments. The diameter of the small holes 1055 may be between about 0.1 mm and about 2 mm.

Through-holes 1056 may be configured to control the passage of the plasma-activated gas (i.e., the ionic, radical, and/or neutral species) through showerhead 1053. For example, the aspect ratio of the holes (i.e., the hole diameter to length) and/or the geometry of the holes may be controlled so that the flow of ionically-charged species in the activated gas passing through showerhead 1053 is reduced. Through-holes 1056 in showerhead 1053 may include a tapered portion that faces chamber plasma region 1020, and a cylindrical portion that faces substrate processing region 1070. The cylindrical portion may be proportioned and dimensioned to control the flow of ionic species passing into substrate processing region 1070. An adjustable electrical bias may also be applied to showerhead 1053 as an additional means to control the flow of ionic species through showerhead 1053.

Alternatively, through-holes 1056 may have a smaller inner diameter (ID) toward the top surface of showerhead 1053 and a larger ID toward the bottom surface. In addition, the bottom edge of through-holes 1056 may be chamfered to help evenly distribute the plasma effluents in substrate processing region 1070 as the plasma effluents exit the showerhead and promote even distribution of the plasma effluents and precursor gases. The smaller ID may be placed at a variety of locations along through-holes 1056 and still allow showerhead 1053 to reduce the ion density within substrate processing region 1070. The reduction in ion density results from an increase in the number of collisions with walls prior to entry into substrate processing region 1070. Each collision increases the probability that an ion is neutralized by the acquisition or loss of an electron from the wall. Generally speaking, the smaller ID of through-holes 1056 may be between about 0.2 mm and about 20 mm. In other embodiments, the smaller ID may be between about 1 mm and 6 mm or between about 0.2 mm and about 5 mm. Further, aspect ratios of the through-holes 1056 (i.e., the smaller ID to hole length) may be approximately 1 to 20. The smaller ID of the through-holes may be the minimum ID found along the length of the through-holes. The cross sectional shape of through-holes 1056 may be generally cylindrical, conical, or any combination thereof.

FIG. 3B is a bottom view of a showerhead 1053 for use with a processing chamber according to embodiments. Showerhead 1053 corresponds with the showerhead shown in FIG. 3A. Through-holes 1056 are depicted with a larger inner-diameter (ID) on the bottom of showerhead 1053 and a smaller ID at the top. Small holes 1055 are distributed substantially evenly over the surface of the showerhead, even amongst the through-holes 1056 which helps to provide more even mixing than other embodiments described herein.

An exemplary patterned substrate may be supported by a pedestal (not shown) within substrate processing region 1070 when fluorine-containing plasma effluents and hydrogen-containing plasma effluents arrive through through-holes 1056 in showerhead 1053. Though substrate processing region 1070 may be equipped to support a plasma for other processes such as curing, no plasma is present during the etching of patterned substrate according to embodiments.

A plasma may be ignited either in chamber plasma region 1020 above showerhead 1053 or substrate processing region 1070 below showerhead 1053. A plasma is present in chamber plasma region 1020 to produce the radical-fluorine from an inflow of the fluorine-containing precursor. An AC voltage typically in the radio frequency (RF) range is applied between the conductive top portion (lid 1021) of the processing chamber and showerhead 1053 to ignite a plasma in chamber plasma region 1020 during deposition. An RF power supply generates a high RF frequency of 13.56 MHz but may also generate other frequencies alone or in combination with the 13.56 MHz frequency.

The top plasma may be left at low or no power when the bottom plasma in the substrate processing region 1070 is turned on to either cure a film or clean the interior surfaces bordering substrate processing region 1070. A plasma in substrate processing region 1070 is ignited by applying an AC voltage between showerhead 1053 and the pedestal or bottom of the chamber. A cleaning gas may be introduced into substrate processing region 1070 while the plasma is present.

The pedestal may have a heat exchange channel through which a heat exchange fluid flows to control the temperature of the substrate. This configuration allows the substrate temperature to be cooled or heated to maintain relatively low temperatures (from room temperature through about 120° C.). The heat exchange fluid may comprise ethylene glycol and water. The wafer support platter of the pedestal (preferably aluminum, ceramic, or a combination thereof) may also be resistively heated to achieve relatively high temperatures (from about 120° C. through about 1100° C.) using an embedded single-loop embedded heater element configured to make two full turns in the form of parallel concentric circles. An outer portion of the heater element may run adjacent to a perimeter of the support platter, while an inner portion runs on the path of a concentric circle having a smaller radius. The wiring to the heater element passes through the stem of the pedestal.

The chamber plasma region or a region in a remote plasma system may be referred to as a remote plasma region. In embodiments, the radical precursors (e.g. radical-fluorine and/or radical-hydrogen) are formed in the remote plasma region and travel into the substrate processing region where the combination preferentially etches tungsten oxide. Plasma power may essentially be applied only to the remote plasma region, in embodiments, to ensure that the radical-fluorine and/or the radical-hydrogen (which together may be referred to as plasma effluents) are not further excited in the substrate processing region.

In embodiments employing a chamber plasma region, the excited plasma effluents are generated in a section of the substrate processing chamber partitioned from the substrate processing region. The substrate processing region, is where the plasma effluents mix and react to etch the patterned substrate (e.g., a semiconductor wafer). The excited plasma effluents may also be accompanied by inert gases (in the exemplary case, argon). The substrate processing region may be described herein as “plasma-free” during etching of the substrate. “Plasma-free” does not necessarily mean the region is devoid of plasma. A relatively low concentration of ionized species and free electrons created within the plasma region do travel through pores (apertures) in the partition (showerhead/ion suppressor) due to the shapes and sizes of through-holes 1056. In some embodiments, there is essentially no concentration of ionized species and free electrons within the substrate processing region. The borders of the plasma in the chamber plasma region are hard to define and may encroach upon the substrate processing region through the apertures in the showerhead. In the case of an inductively-coupled plasma, a small amount of ionization may be effected within the substrate processing region directly. Furthermore, a low intensity plasma may be created in the substrate processing region without eliminating desirable features of the forming film. All causes for a plasma having much lower intensity ion density than the chamber plasma region (or a remote plasma region, for that matter) during the creation of the excited plasma effluents do not deviate from the scope of “plasma-free” as used herein.

Nitrogen trifluoride (or another fluorine-containing precursor) may be flowed into chamber plasma region 1020 at rates between about 5 sccm and about 500 sccm, between about 10 sccm and about 300 sccm, between about 25 sccm and about 200 sccm, between about 50 sccm and about 150 sccm or between about 75 sccm and about 125 sccm in embodiments. Ammonia (NH₃) may be flowed into chamber plasma region 1020 at rates greater than or about 50 sccm, greater than or about 100 sccm, greater than or about 200 sccm, or greater than or about 500 sccm according to embodiments.

Combined flow rates of fluorine-containing precursor and hydrogen-containing precursor (ammonia) into the chamber may account for 0.05% to about 20% by volume of the overall gas mixture; the remainder being carrier gases. The fluorine-containing precursor and the hydrogen-containing precursor are flowed into the remote plasma region but the plasma effluents have the same volumetric flow ratio, in embodiments. In the case of the fluorine-containing precursor, a purge or carrier gas may be first initiated into the remote plasma region before those of the fluorine-containing gas to stabilize the pressure within the remote plasma region.

Plasma power applied to the remote plasma region can be a variety of frequencies or a combination of multiple frequencies. In the exemplary processing system the plasma is provided by RF power delivered between lid 1021 and showerhead 1053. In an embodiment, the energy is applied using a capacitively-coupled plasma unit. When using a Frontier™ or similar system, the remote plasma source power may be between about 100 watts and about 3000 watts, between about 200 watts and about 2500 watts, between about 300 watts and about 2000 watts, or between about 500 watts and about 1500 watts in embodiments. The RF frequency applied in the exemplary processing system may be low RF frequencies less than about 200 kHz, high RF frequencies between about 10 MHz and about 15 MHz or microwave frequencies greater than or about 1 GHz according to embodiments.

Substrate processing region 1070 can be maintained at a variety of pressures during the flow of carrier gases and plasma effluents into substrate processing region 1070. The pressure within the substrate processing region is below or about 50 Torr, below or about 30 Torr, below or about 20 Torr, below or about 10 Torr or below or about 5 Torr in embodiments. The pressure may be above or about 0.1 Torr, above or about 0.2 Torr, above or about 0.5 Torr or above or about 1 Torr according to embodiments. Lower limits on the pressure may be combined with upper limits on the pressure in embodiments.

In one or more embodiments, the substrate processing chamber 1001 can be integrated into a variety of multi-processing platforms, including the Producer™ GT, Centura™ AP and Endura™ platforms available from Applied Materials, Inc. located in Santa Clara, Calif. Such a processing platform is capable of performing several processing operations without breaking vacuum. Processing chambers that may implement embodiments of the present invention may include dielectric etch chambers or a variety of chemical vapor deposition chambers, among other types of chambers.

Embodiments of the deposition systems may be incorporated into larger fabrication systems for producing integrated circuit chips. FIG. 4 shows one such system 1101 of deposition, baking and curing chambers according to embodiments. In the figure, a pair of FOUPs (front opening unified pods) 1102 supply substrate substrates (e.g., 300 mm diameter wafers) that are received by robotic arms 1104 and placed into a low pressure holding areas 1106 before being placed into one of the wafer processing chambers 1108 a-f. A second robotic arm 1110 may be used to transport the substrate wafers from the low pressure holding areas 1106 to the wafer processing chambers 1108 a-f and back. Each wafer processing chamber 1108 a-f, can be outfitted to perform a number of substrate processing operations including the dry etch processes described herein in addition to cyclical layer deposition (CLD), atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), etch, pre-clean, degas, orientation and other substrate processes.

The wafer processing chambers 1108 a-f may include one or more system components for depositing, annealing, curing and/or etching a flowable dielectric film on the substrate wafer. In one configuration, two pairs of the processing chamber (e.g., 1108 c-d and 1108 e-f) may be used to deposit dielectric material on the substrate, and the third pair of processing chambers (e.g., 1108 a-b) may be used to etch the deposited dielectric. In another configuration, all three pairs of chambers (e.g., 1108 a-f) may be configured to etch a dielectric film on the substrate. Any one or more of the processes described may be carried out on chamber(s) separated from the fabrication system shown in embodiments.

The substrate processing system is controlled by a system controller. In an exemplary embodiment, the system controller includes a hard disk drive, a floppy disk drive and a processor. The processor contains a single-board computer (SBC), analog and digital input/output boards, interface boards and stepper motor controller boards. Various parts of CVD system conform to the Versa Modular European (VME) standard which defines board, card cage, and connector dimensions and types. The VME standard also defines the bus structure as having a 16-bit data bus and a 24-bit address bus.

System controller 1157 is used to control motors, valves, flow controllers, power supplies and other functions required to carry out process recipes described herein. A gas handling system 1155 may also be controlled by system controller 1157 to introduce gases to one or all of the wafer processing chambers 1108 a-f. System controller 1157 may rely on feedback from optical sensors to determine and adjust the position of movable mechanical assemblies in gas handling system 1155 and/or in wafer processing chambers 1108 a-f. Mechanical assemblies may include the robot, throttle valves and susceptors which are moved by motors under the control of system controller 1157.

In an exemplary embodiment, system controller 1157 includes a hard disk drive (memory), USB ports, a floppy disk drive and a processor. System controller 1157 includes analog and digital input/output boards, interface boards and stepper motor controller boards. Various parts of multi-chamber processing system 1101 which contains substrate processing chamber 1001 are controlled by system controller 1157. The system controller executes system control software in the form of a computer program stored on computer-readable medium such as a hard disk, a floppy disk or a flash memory thumb drive. Other types of memory can also be used. The computer program includes sets of instructions that dictate the timing, mixture of gases, chamber pressure, chamber temperature, RF power levels, susceptor position, and other parameters of a particular process.

A process for etching, depositing or otherwise processing a film on a substrate or a process for cleaning chamber can be implemented using a computer program product that is executed by the controller. The computer program code can be written in any conventional computer readable programming language: for example, 68000 assembly language, C, C++, Pascal, Fortran or others. Suitable program code is entered into a single file, or multiple files, using a conventional text editor, and stored or embodied in a computer usable medium, such as a memory system of the computer. If the entered code text is in a high level language, the code is compiled, and the resultant compiler code is then linked with an object code of precompiled Microsoft Windows® library routines. To execute the linked, compiled object code the system user invokes the object code, causing the computer system to load the code in memory. The CPU then reads and executes the code to perform the tasks identified in the program.

The interface between a user and the controller may be via a touch-sensitive monitor and may also include a mouse and keyboard. In one embodiment two monitors are used, one mounted in the clean room wall for the operators and the other behind the wall for the service technicians. The two monitors may simultaneously display the same information, in which case only one is configured to accept input at a time. To select a particular screen or function, the operator touches a designated area on the display screen with a finger or the mouse. The touched area changes its highlighted color, or a new menu or screen is displayed, confirming the operator's selection.

As used herein “substrate” may be a support substrate with or without layers formed thereon. The patterned substrate may be an insulator or a semiconductor of a variety of doping concentrations and profiles and may, for example, be a semiconductor substrate of the type used in the manufacture of integrated circuits. Exposed “silicon nitride” of the patterned substrate is predominantly Si₃N₄ but may include minority concentrations of other elemental constituents (e.g. oxygen, hydrogen, carbon). Exposed “silicon oxide” of the patterned substrate is predominantly SiO₂ but may include minority concentrations of other elemental constituents (e.g. nitrogen, hydrogen, carbon). In some embodiments, silicon oxide films etched using the methods described herein consist essentially of silicon and oxygen. “Tungsten oxide” is predominantly tungsten and oxygen but may include minority concentrations of other elemental constituents (e.g. nitrogen, hydrogen, carbon). Tungsten oxide may consist of tungsten and oxygen. “Titanium nitride” is predominantly titanium and nitrogen but may include minority concentrations of other elemental constituents (e.g. nitrogen, hydrogen, carbon). Titanium nitride may consist of titanium and nitrogen.

The term “precursor” is used to refer to any process gas which takes part in a reaction to either remove material from or deposit material onto a surface. “Plasma effluents” describe gas exiting from the chamber plasma region and entering the substrate processing region. Plasma effluents are in an “excited state” wherein at least some of the gas molecules are in vibrationally-excited, dissociated and/or ionized states. A “radical precursor” is used to describe plasma effluents (a gas in an excited state which is exiting a plasma) which participate in a reaction to either remove material from or deposit material on a surface. “Radical-fluorine” (or “radical-hydrogen”) are radical precursors which contain fluorine (or hydrogen) but may contain other elemental constituents. The phrase “inert gas” refers to any gas which does not form chemical bonds when etching or being incorporated into a film. Exemplary inert gases include noble gases but may include other gases so long as no chemical bonds are formed when (typically) trace amounts are trapped in a film.

The terms “gap” and “trench” are used throughout with no implication that the etched geometry has a large horizontal aspect ratio. Viewed from above the surface, trenches may appear circular, oval, polygonal, rectangular, or a variety of other shapes. A trench may be in the shape of a moat around an island of material. The term “via” is used to refer to a low aspect ratio trench (as viewed from above) which may or may not be filled with metal to form a vertical electrical connection. As used herein, a conformal etch process refers to a generally uniform removal of material on a surface in the same shape as the surface, i.e., the surface of the etched layer and the pre-etch surface are generally parallel. A person having ordinary skill in the art will recognize that the etched interface likely cannot be 100% conformal and thus the term “generally” allows for acceptable tolerances.

Having disclosed several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosed embodiments. Additionally, a number of well-known processes and elements have not been described to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included.

As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the dielectric material” includes reference to one or more dielectric materials and equivalents thereof known to those skilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups. 

The invention claimed is:
 1. A method of etching tungsten oxide, the method comprising: flowing a fluorine-containing precursor and ammonia (NH₃) into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead while forming a remote plasma in the remote plasma region to produce plasma effluents; and etching the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead.
 2. The method of claim 1 wherein the tungsten oxide comprises about 20% or more tungsten and about 60% or more oxygen.
 3. The method of claim 1 wherein a flow rate of the ammonia (NH₃) is between about 75% and about 100% of the flow rate of the fluorine-containing precursor.
 4. The method of claim 1 wherein the substrate is a patterned substrate which further comprises one or more exposed regions of tungsten, silicon nitride, silicon oxide or titanium nitride.
 5. The method of claim 4 wherein the operation of etching the tungsten oxide comprises etching tungsten oxide faster than tungsten by a ratio of about 10:1 or more, faster than silicon nitride by a ratio of about 20:1 or more, faster than silicon oxide by a ratio of about 15:1 or more or faster than titanium nitride by a ratio of about 20:1 or more.
 6. The method of claim 1 wherein a temperature of the substrate is greater than or about 30° C. and less than or about 300° C. during the etching operation.
 7. A method of etching tungsten oxide, the method comprising: flowing a fluorine-containing precursor and ammonia (NH₃) into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead, wherein the fluorine-containing precursor and the ammonia are flowed at a first flow rate ratio (ammonia:fluorine-containing precursor); forming a remote plasma in the remote plasma region to produce plasma effluents from the fluorine-containing precursor and the ammonia (NH₃); etching a first portion of the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead; increasing flow rate ratio (ammonia:fluorine-containing precursor) to a second flow rate ratio which is greater than the first flow rate ratio; etching a second portion of the tungsten oxide from the substrate by continuing to flow plasma effluents into the substrate processing region.
 8. The method of claim 7 wherein the first portion of the tungsten oxide has a W:O atomic ratio of roughly 1:3.
 9. The method of claim 7 wherein the second portion of the tungsten oxide has a W:O atomic ratio of roughly 1:2.
 10. The method of claim 7 wherein the second flow rate ratio exceeds the first flow rate ratio by at least 1%.
 11. The method of claim 7 wherein the operation of increasing flow rate ratio comprises changing from the first flow rate ratio to the second flow rate ratio essentially in a single discrete step.
 12. A method of etching tungsten oxide, the method comprising: flowing nitrogen trifluoride (NF₃) at a nitrogen trifluoride flow rate and ammonia (NH₃) at a first ammonia flow rate into a remote plasma region fluidly coupled to a substrate processing region via through-holes in a showerhead; forming a remote plasma in the remote plasma region to produce plasma effluents from the nitrogen trifluoride and the ammonia; etching a first portion of the tungsten oxide from a substrate disposed within the substrate processing region by flowing the plasma effluents into the substrate processing region through the through-holes in the showerhead; increasing the flow rate of the ammonia from the first ammonia flow rate to a second ammonia flow rate; etching a second portion of the tungsten oxide from the substrate by continuing to flow plasma effluents into the substrate processing region.
 13. The method of claim 12 wherein the first ammonia flow rate is between 75% and 92% of the nitrogen trifluoride flow rate.
 14. The method of claim 12 wherein the second ammonia flow rate is between 90% and 97% of the nitrogen trifluoride flow rate.
 15. The method of claim 12 wherein forming the remote plasma in the remote plasma region to produce plasma effluents comprises applying RF power between about 10 watts and about 2500 watts to the remote plasma region. 